An important application is the pre-operative selection of intraocular lenses in the treatment of cataracts. The most important measurement for this selection is the axial length of the eye from the front of the cornea to the retina. In the prior art this is preferably carried out using non-contact optical interferometry processes, which are known as PCI (partial coherence interferometry) or OCT (optical coherence tomography). In these processes, structural interfaces can be represented as one-dimensional depth profiles (A-scans) or as two-dimensional cross-sectional views (B-scans), specular reflexes on the optical boundary surfaces and/or light that is scattered in the different media of the eye being detected.
In both measurement processes it is important for the measurement to be taken along an axially oriented axis corresponding to the visual axis. Otherwise, mistakes can arise during the choice of IOL so that the patient's vision is significantly impaired after implantation of the IOL.
In order to guarantee, with a high level of reliability, that the measurement is along the visual axis, in the prior art, the patient is offered a fixation light to fixate on while the measurement is being taken with the optical measuring instrument. This aligns the visual axis of the eye with the main measuring axis of the instrument (instrument axis), which also corresponds to the Z-axis of the measuring instrument's coordinate system. This can be found in the literature (ISO/CD 19980, “Ophthalmic instruments—Corneal topographers.” 2009). If the instrument axis is aligned with the visual axis, then, in most cases, the cornea and the retina are sufficiently vertical to the main measuring axis, so that the measuring beams reflected by the cornea and the retina are accurately registered by the measuring instrument.
According to a first method described in the literature (W. Haigis, “Optical Coherence Biometry,” in Modern Cataract Surgery, T. Kohnen, Ed. Basel: Karger Publishers, 2002, pp. 119-130), the axial length is measured by partial coherence interferometry using the double-path method. In this method two beams with different optical path lengths fall into the eye and are specularly reflected at the front of the cornea and the retina to produce interference. The eye length can be determined from the signals at different optical path lengths. Since a usable signal is only obtained if there is a specular reflex from both the cornea and the retina, this process has the advantage that the cornea and the retina must be approximately vertical to the measuring beam and therefore to the instrument axis in order to generate a distance signal.
It has been demonstrated experimentally that, under these measuring conditions which produce a usable distance signal, the instrument axis/measuring axis is approximately identical to the visual axis and corresponds to the axial length distance measured along the instrument axis, which is crucial for calculating the IOL.
This measuring process therefore virtually excludes the possibility of obtaining a false reading for eye length, if the optical axis deviates too much from the instrument axis, and then using this for calculating the IOL.
However, a disadvantage is that it relies upon a minimum amount of cooperation from the patient to fixate his/her gaze during the measurement and, if this is not forthcoming, no measurements, or very few, and therefore statistically less valid, readings of the axial eye length, can be determined
Another disadvantage is that it is difficult to obtain B-scan readings or anterior chamber depth measurements, since, because of the angle of the measuring beam relative to the interfaces, either the cornea or the lens fails to produce a specular reflex that can be registered by the device in these measurements. Therefore newer methods, which promise a higher degree of reliability in the selection of intraocular lenses and require measurement of the anterior chamber depth, lens thickness or lens radius, cannot be used or can only be used with difficulty.
According to a second method described in the literature (Haag-Streit AG, “Biometry Connected . . . ” June 2010), the intraocular distances are measured by means of one or more so-called B-scans, obtained by optical coherence tomography. This can be used to resolve not only the front surface of the cornea and the retina but also other tissue structures. For example, cornea thickness, anterior chamber depth and/or lens thickness can be determined.
The basic principle of the OCT method, described for example in U.S. Pat. No. 5,321,501 A, is based on white light interferometry and compares the duration of a signal using an interferometer (usually a Michelson or Mach-Zehnder interferometer). There the arm of known optical path length is used as an object-external reference to the measuring arm. The interference of the signals from both arms produces a pattern, from which the relative optical path length within an A-scan (single depth signal) can be deduced. In a one-dimensional raster scan, the beam is then directed transversely in one or two directions, allowing the recording of a two-dimensional B-scan or a three-dimensional tomogram. This produces sufficient signals even in the B-scan, because this process records both specular reflexes and also diffusion in the object.
However, unlike the double path method, the measuring principle of these processes does not in itself guarantee that the axial length (axial length of the eye) will be measured along the correct axis (visual axis), which is important for calculating the intraocular lenses. This is because a recording and a signal can still be obtained, even though the measuring beam is not vertically incident upon the front surface of the cornea or not aligned with the visual axis. Measurement along the instrument axis then provides an A-scan, which in itself does not appear to be defective, even if it was not measured along the visual axis due to poor fixation. However, in general, deriving the axial length from the measurement along the instrument axis would result in incorrect, systematically shortened readings, since, if the measuring instrument is not properly aligned with the visual axis because of eye movement or poor fixation, the A-scan measures too far off the visual axis and, with a typically convex eye, this results in a shortening of the cornea to retina distance.
Generally there is also the problem of the lateral matching of the B-scan to the eye. If the eye is moved during the measurement itself or even during alignment of the measuring instrument on the eye, this results in incorrect measurements due to inaccurate alignment.
If these eye movements are not taken into consideration, a B-scan and the intraocular distances derived from it are laterally displaced relative to the eye and are therefore incorrectly assigned. There is therefore no guarantee that the A-scan measures along the instrument axis or that the A-scan within a B-scan running along the instrument axis actually measures the eye length. Moreover, even if they are accurately aligned, only a few A-scans—that is to say only those along the instrument axis—can be used for calculating the axial length, so that the measured axial length is associated with a relatively high degree of statistical uncertainty.
A further process for determining the distances between localised interfaces in the eye is described in DE 10 2010 051 281 A1. Using the scans taken under different conditions, which scans include at least two of the interfaces present in the eye, a parametric eye model can be appropriately adjusted by a control and evaluation unit to allow model-based determination of the eye biometry.
However, even with this solution, the automatic evaluation of A and B-scans to obtain biometric data is faced with the problem of a large number of measuring situations and disturbances. These include, for example, attenuation of the measuring beam by cataracts or defocusing of the measuring beam due to refractive errors or the presence of pathological conditions.